Rotating element display material has been disclosed in U.S. Pat. Nos. 4,126,854, 4,143,103 and 5,389,945, each herein incorporated by reference. The rotating element display material generally comprises a substrate, a dielectric fluid and a set of rotatable elements. The rotating element display material has found use as reusable “electric paper” since it has many of the attributes of paper documents when the display is in sheet form. The rotating elements are embedded in an elastomer material which is then cured and swelled to create cavities around the elements. The rotating element display material has many of the qualities of real paper, including being flexible and affordable, it can be written on like paper, it can be copied like paper, it can be easily viewed in high levels of ambient light, and it will retain images nearly as long as regular paper.
The rotating elements, also referred to as bichromal spheres or bichromal balls, are optically and electronically anisotropic. For example, a rotating element has two distinct hemispheres, one black and one white, with each hemisphere having a distinct electrical characteristic giving it a dipole moment with respect to a dielectric fluid. An image is formed on the electric paper by applying an external electric field which rotates the bichromal sphere differentiated by the contrasting charges of the two hemispheres of the rotating element. The difference in charges between the hemispheres of the ball causes the ball to act like a dipole in the presence of an electrical field, which in turn causes the ball to rotate until its dipole vector lines up in the direction of the externally applied electric field.
Varying methods have been developed for the fabrication of the rotating elements; however, these existing methods are not adaptable to either a continuous fabrication of the rotating elements or a large-scale fabrication process which is commercially acceptable. For example, U.S. Pat. No. 4,438,160 describes the fabrication of rotating elements using a vapor deposition method. Generally, the vapor deposition method entails fabrication of rotating elements of a single color. A contrasting pigment is then evaporated on to one side of the rotational element resulting in a bichromal sphere.
Alternately, fabrication of rotating elements, as described in U.S. Pat. No. 4,810,431 to Leidner, may be achieved by (a) co-extruding a fiber of a semi-circular layer of a polyethylene pigmented white and a semi-circular black layer of polyethylene containing magnetite; (b) chopping the resulting fiber into fine particles ranging from 10 microns to about 10 millimeters; (c) mixing the particles with clay or anti-agglomeration materials, and (d) heating the mixture with a liquid at about 120° C. to spheradize the particles, followed by cooling to allow of solidification.
U.S. Pat. No. 5,262,098 describes an apparatus useful for fabricating bichromal spheres which comprises a separator member, also called a spinning disk, having opposing first and second surfaces, an outer edge region in contact with both surfaces, and means for delivering a first and second pigmented polymer melt over the first and second surfaces, respectively. The respective liquid materials arrive at the edge at substantially the same flow rate and form a reservoir outboard of the edge region. The reservoir comprises side by side regions of different colors which do not intermix. The separator member is rotated and the centrifugal forces cause the first and second liquid materials to pull away from the separator member and out of the reservoir as a plurality of side-by-side bichromal streams. As a result of the centrifugal forces, the forward ends of the bichromal steams become unstable, break into droplets which form into spherical balls each comprising hemispheres of differently pigmented polymer melts.
The existing systems for fabricating bichromal spheres employ a batch process for the preparation of the pigmented polymer melt. Basically, polymer material forming the matrix of the sphere, such as polyethylene wax, is poured into mixing tanks and heated. A separate tank is used for making the white pigmented polymer and for making the black pigmented polymer. A white pigment, usually titanium dioxide, is added to the first tank of molten material, mixed for approximately two hours then moved to solidify. Similarly, a black pigment, such as ferrous oxide, and optionally a charge control agent is added to the second tank of molten material which is also mixed for approximately two hours and allowed to solidify. Next, the white pigmented polymer material and the black pigmented polymer material are placed into separate melters where a white pigmented melt and black pigmented melt are formed. The separate melts are then introduced to the spinning disk as described above.
The inefficiencies of the existing methods for fabricating the rotational elements make these methods ineffective for continuous or large-scale production processes. For example, the existing fabrication methods which utilize spinning disks require 3 to 4 hours, or more, depending on batch size, to prepare and produce a single batch of rotating elements. However, actual production of useable rotating elements occurs for as little as 20 minutes, with the remainder being spent on preparation of the material and stabilization of the system. In addition, during the stabilization of the system, the production of irregular and unusable rotational elements results in waste of raw materials. Finally, rotational elements produced by different batches may have inconsistencies in critical features of the sphere such as contrast ratio of the display sheet into which the spheres are incorporated, diameter, complimentarity, and sphere quality, where sphere quality is understood to be a measure of the roundness of the spheres.
Accordingly, it is an object of the present invention to provide for a continuous fabrication of anisotropic rotational elements, such as hemispherically bichromal spheres, wherein a high level of consistency and complimentarity of the rotational element is obtained.